I gather you're American given Florida as your home state; in that case, no visa required, just the US passport you enter Argentina with.

Most ferries leave from Puerto Madero dock in Buenos Aires, downtown. For a day trip, the only logical place to do in Uruguay would be Colonia although the ferry companies do sell a one-day package tour to Montevideo.

No visa but you have to fill out the usual tourist card that one has to fill out when entering any foreign country. Keep it with you at all times. You have to pay a tax as you leave. It is something like $8. I don't remember exactly. It isn't much but you must produce that piece of paper you entered with and your passport. I can't remember if they take credit cards since we paid in Uruguayan pesos as we left. It is a good idea to keep the exit fee in cash (efectivo) in case they don't take credit cards. We have run into that scenario even in the US when we returned, believe it or not.